1248 



Journal of Applied Microscopy 



Remarks. — If much ammonium chloride is 

 present, the crystals at first formed will rapidly 

 disappear, or there may be no separation of the 

 calcium salt owing to its marked solubility in 

 solutions of ammonium chloride. 



The double ammonium arsenates are iso- 

 morphous with the double ammonium phosphates, 

 a fact which is liable to give rise to errors in the 

 interpretation of results. Moreover it happens 

 that the usefulness of this elegant reaction is 

 unfortunately restricted, since the elements of 

 the magnesium group, which are often present 

 in mixtures to be tested for calcium, unite to 

 form double ammonium arsenates of like crystal- 

 line appearance. 

 Strontium forms minute stars and tiny crystalline grains, while barium yields 

 a dense precipitate amorphous in appearance. 



Exercises for Practice. 



Try the above reaction on salts of calcium, strontium and barium, first alone, 

 then in mixtures. 



Try on salts of magnesium, zinc and calcium. 



Try a salt of calcium in the presence of much ammonium chloride. 



VI. Priinary Sodium Carbonate added to solutions containing Calcium causes 

 the separation of crystalline Calcium Carbonate. 



CaCl., 4- 2HNaC03 = CaCOg + 2NaCl ; CO, + H2O. 



Method. — Cause a concentrated solution of the reagent to flow into a drop of 

 a dilute neutral, or ammoniacal, solution of the calcium salt. In a short time 

 very small disks and rhombs of the compound CaCOg appear. 



Remarks. — The addition of the reagent in solid form gives nearly as good 

 results. 



Warming the preparation increases the rapidity of tiie reaction and leads to 

 the formation of better crystals. 



Unless the test drop is quite dilute an amorphous precipitate results. 



Ammonium carbonate can be substituted for the sodium salt, the crystals then 

 differ but little if any from those obtained as above. Normal sodium carbonate 

 produces an amorphous precipitate. 



Strontium is precipitated in the form of dumb-bell shaped aggregates and in 

 the form of " sphero-crystals." Barium gives forms somewhat similar in 

 appearance. 



Elements of the magnesium group interfere. Lithium likewise interferes. 

 But the chlorides of iron and aluminum and the salts of boric acid have n9 appre- 

 ciable effect on the reaction. 



When in doubt as to the nature of a precipitate formed by the treatment with 



